Traditionally, polymers and commodity chemicals have been prepared from petroleum-derived feedstocks. However, as petroleum reservoirs are rapidly depleting and concomitantly becoming more difficult to access, an exigency to develop renewable or “green” alternative materials from biologically-derived resources has been at the vanguard of much current research, particularly in the role of commercially tenable surrogates to conventional, petroleum-derived counterparts, or for generating the same materials as produced from fossil, non-renewable sources.
One of the most abundant kinds of biologically-derived or renewable alternative feedstock for such materials is carbohydrates. Carbohydrates, however, are generally unsuited to current high temperature industrial processes. In contrast to petroleum-based hydrophobic aliphatic or aromatic feedstocks with a low degree of functionalization, carbohydrates such as sugars are complex, multi-functionalized hydrophilic materials. As a consequence, researchers have sought to produce biologically-based chemicals that originate from carbohydrates, but which are less highly functionalized, including more stable bi-functional compounds such as 2,5-furandicarboxylic acid (FDCA), levulinic acid, and 1,4:3,6-dianhydrohexitols.
1,4:3,6-dianhydrohexitols (also referred to herein as isohexides) are derived from renewable resources such as cereal-based polysaccharides and the sugars obtained by hydrolysis thereof. Isohexides are a class of bicyclic furanodiols that are derived from the dehydration of corresponding reduced sugar alcohols, for example, D-sorbitol, D-mannitol, and D-iditol are dehydrated and cyclized to A) isosorbide, B) isomannide, and C) isoidide, respectively, the structures of which are illustrated in Scheme 1.
The dehydration of sugar alcohols to isohexides by conventional techniques such as use of sulfuric acid typically results in the production of a number of undesirable byproducts. The separation of such byproducts from the desired isohexides as well as other furanic-derivatives remains complicated and costly. Furthermore, the by-products present an undesirable yield loss. Thus, a process that can generate fewer byproducts and higher yields of the desired product, as well as being better in compositional accountability would be welcome.